/**
*      author: Y_Yao
*      created: 2024-09-27 21:18:51
**/

#include<bits/stdc++.h>

using namespace std;
using LL = long long;
const int INF = 0x3f3f3f3f;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int a,b,c,d;
    cin >> a >> b >> c >> d;

    LL m = a * (b - 1);
    LL n = c * (d - 1);
    if(n > m) cout << "YES" <<endl;
    else cout << "NO" << endl;
    return 0;
}